PLANTS 41 



fer of pollen, preferably by means of cross-pollination. Hence, it is 

 the basis of the many and varied coactions exhibited by the flower- 

 loving insects and birds. When plants are one-flowered, it is obvious 

 that cross-pollination alone is possible, while in such clusters as the 

 head of composites, the automatic transfer of pollen between florets 

 may become a regular occurrence. The relative position and develop- 

 ment of the stamens and pistils not only affect the method of transfer, 

 but they also determine the kind of fertilization that results. A de- 

 tailed treatment of the various types of pollination is to be found 

 in Knuth's "Handbook" (pp. 28-60) and various other works. 



Period of Flowering. The time and duration of flowering bear a 

 fairly definite relation to the adult stage of the individual plant. 

 However, in many perennial herbs and woody plants, the relation is 

 inverted, the flowers appearing before or with the leafy shoot, owing 

 apparently to a delayed bud formation overtaken by winter. A few 

 species blossom more or less throughout the growing period and others 

 are somewhat irregular, but the large majority fall within a fairly 

 definite season. As a consequence, it is possible to distinguish these 

 as prevernal, vernal, estival, autumnal, and hiemal plants, each giving 

 character to a particular aspect. 



While the flowers of most species open once for all, in a consider- 

 able number there is a marked daily rliythm of opening and closing. 

 This cycle depends primarily upon temperature, but in some cases it 

 is connected with light, as radiant energy, or with humidity. The 

 part actively concerned in the movement is the corolla, the calyx 

 assuming an imitative role; in composites, the involucre may be as 

 much affected as the ray-florets. Although opening and closing may 

 occur at almost any time during the twenty-four hours, the tendency 

 is to open in early morning or evening. Species that open during 

 the daytime are termed hemcranthous or day bloomers, in contrast to 

 nyctanthous or night bloomers. 



Flower Cycles. AVithin the seasonal cycle of blooming of each 

 species lies the life cycle of the individual flower, essentially alike 

 for all, except where there are two or more kinds of flowers, such as 

 perfect, staminate and pistillate, or open and cleistogamous. In all 

 flowers or heads with movement, there is also a daily cycle as just 

 indicated, with the exception of ephemerals, in which the life period 

 and the daily cycle coincide (Fig. 2). 



It is evident that the seasonal and the daily period of flowers 

 are intimately connected with the coactions of insects, esi)ecially pol- 

 linating ones. Practically every change of the flower or its parts has 

 some influence upon the procedure in attracting insects and insuring 



